In recent years, mobile terminals (cell phones, handheld terminals, etc.) with imaging devices are becoming common. Solid-state image sensors are used for imaging devices. Solid-state image sensors include CCD (Charge Coupled Device) image sensors and CMOS (Complementary Metal Oxide Semiconductor) image sensors, etc. Generally, an imaging device used in a mobile terminal includes a small solid-state image sensor at a low pixel count compared to a normal digital camera, etc., and a single-focus optical system composed of around 1 to 4 plastic lenses. Moreover, there are various demands for such imaging devices. For example, there is demand for the ability to support imaging elements with high pixel counts, or the ability to capture images at long and short distances. In particular, to enable image capture at short distances, there is demand for variable-magnification optical systems that are small enough to be mounted on a mobile terminal but also have a wide angle.
In many thin mobile terminals, bending optical systems that use a reflective optical element such as a prism to bend the optical axis 90° are used. Among variable-magnification optical systems including four lens groups that respectively have a negative, positive, negative and positive refractive power in that order from the object side, variable-magnification optical systems designed to be thinner by applying reflective optical elements in the first lens group having a negative refractive power are known (refer to Patent documents 1 and 2).